1. Field of the Invention
The present invention relates to a melt transesterification process for the production of polycarbonates. More particularly, the invention relates to a melt transesterification process of reacting a bis(aryl)carbonate and a dihydric phenol in the presence of a heterocyclic borate salt catalyst.
2. Discussion of the Background
The conventional method for the production of polycarbonates is an interfacial polymerization route employing phosgene as a raw material. Because of its toxicity and difficulties in transportation, phosgene causes considerable safety problems. It is usually manufactured on-site in a dedicated plant and transferred by pipeline to the vessel where it is to be used. Toxicity considerations notwithstanding, when phosgene is used, chlorine, obtained as a by-product, is converted to sodium chloride, the disposal of which presents ecological problems.
Thus, considerable effort has been devoted to replacing phosgene in polycarbonate manufacturing. A non-phosgene route of particular interest involves the production of dimethyl carbonate (DMC) by oxidative carbonylation of methanol with carbon monoxide and oxygen (U.S. Pat. No. 4,218,391, U.S. Pat. No. 4,318,862 and U.S. Pat. No. 3,963,586). The DMC is converted to a bis(aryl)carbonate by reaction with the corresponding phenol. For instance, diphenyl carbonate (DPC) is obtained by reaction of DMC with phenol (U.S. Pat. No. 4,554,110 and U.S. Pat. No. 4,410,464).
The preparation of aromatic polycarbonates from a bis(aryl)carbonate and a dihydric phenol, such as bisphenol-A (BPA), is performed by means of a transesterification process, which takes place in the melted state in the presence of a catalyst. In U.S. Pat. No. 4,330,664, hydrides of aluminum or boron are used for this purpose; particularly preferred are tetramethylammonium borohydride and lithium aluminum hydride. Japanese Application 62-158719 describes the preparation of polycarbonate by a transesterification route in the presence of KBH.sub.4 as catalyst. In an alternative method, Japanese Application 62-169821 discloses a process in which a quaternary ammonium borohydride compound of formula R'.sub.4 NBH.sub.4 is employed, where R' is a C.sub.1 -C.sub.4 alkyl, a C.sub.6 -C.sub.10 aryl or a C.sub.6 -C.sub.10 arylalkyl group.